This invention relates to a device for detecting electric waveforms which are functionally related to physiological characteristics such as the electrocardiograph (ECG), blood pressure and respiration of a patient. Two uses of the invention will be illustrated and described, the first being for detecting the R-wave or QRS complex of the ECG and the second being for detecting systolic and diastolic blood pressure.
R-wave detectors used in the past generally fall into two classes. The first employs a notch filter and is based on the principle that the QRS complex is rich in 10 to 17 Hz frequency components and that the ECG waveform can be passed through a filter which has a center frequency of about 10 Hz so that the accentuated frequency can be detected. A problem with this class of detectors is that the T-wave of the ECG signal and other bioelectric muscle noise often present in the waveforms of critically ill patients contain components with about the same frequency range so it is difficult for the detector to distinguish them from a true R-wave or QRS complex. Moreover, the QRS portion of the ECG waveform with certain types of heart defects is much wider than the normal or average width for a healthy subject so it is also rich in frequencies lower than the center frequency of the filter which is set for the normal QRS complex. In addition, the peak-to-peak amplitude of a QRS complex can vary between patients and within a single patient by a factor of 25 to 1 which precludes use of an effective automatic gain control since such control usually cannot cover this wide variation and still be linear.
Another class of R-wave detectors operates on the principle that the slope of the leading and trailing edges of the QRS complex are uniquely different from those of the P and T wave portions of the ECG. The assumption is, therefore, that the derivative of the ECG waveform can be obtained and that when the output exceeds some preset threshold value, the equivalent of some preset slope, that this can be detected. The disadvantage of prior derivative class detectors is that some technique must be used to limit the slew rate of the amplifier prior to the detector circuit or muscle spikes and artificial electronic pacemaker pulses which are often present will have slopes equal to or greater than that of the QRS complex. Such similar slopes are hard to distinguish from the R-wave slopes. This class of detectors also cannot be compensated effectively with automatic gain control since the amplitude variation range is very great and the control will not respond in a linear fashion throughout a wide enough range.
Detection of other physiological waveforms or signals present similar problems. The blood pressure signal is an example. The rising edge of this waveform has a characteristic slope which is used to detect the arterial blood pressure signal. When the upslope is detected, the past minimum point is sampled and called the diastolic pressure and the following peak is sampled and called the systolic pressure. In detection of systolic upslope with presently available equipment, an integration of the original pressure waveform is compared with the unintegrated waveform to detect the arterial blood pressure signal. When the upslope is detected, the diastolic and systolic points are sampled. Since the peak and valley detection is done with diodes in an open loop scheme, the voltage drop produced by these diodes must be compensated for in the sample and hold circuit, thus requiring an adjustment. Two basic disadvantages of this class of detectors are: 1) the detection of the upslope is such that low pulse pressures such as pulmonary arterial pressure are missed, thus leaving the peak and valley detection to automatically update; and 2) since the diode drops are evident in the systolic and diastolic detectors, a change in slope will cause a different diode drop, thus changing the peak voltage detected.
Another technique for detection of the systolic and diastolic pressure is to discharge two capacitors used for peak detection alternately such that one or the other capacitor maintains the peak voltage of the systolic pressure at all times. The same technique is used for valley detection. No systolic upslope detector is used. This approach has the following disadvantaes: 1) without the use of the systolic upslope detector, accurate measurement of the systolic and diastolic points are hard to obtain; 2) if a noise spike causes one of the two capacitors to charge to an erroneous level, it will be held until that capacitor is discharged which could take as long as 5 seconds; and 3) with the loss of a signal, it may take as long as one complete discharge cycle to identify the problem and this could take 5 seconds.